Air-source heat pump

ABSTRACT

An air source heat pump wherein heat is extracted from air at ambient temperature in an evaporator ( 4 ) and heat is rejected at a higher temperature in a condenser ( 2 ); further comprises an auxiliary heater ( 7 ) arranged to preheat the ambient air when required to prevent deposition of frost on the evaporator. The combustion heater may be a gas fired boiler, arranged to heat recirculating water for supplying heating radiators and wherein the flue gases mix with the ambient air before it passes over the evaporator.

The present invention relates to air-source heat pumps, which extract heat from air at ambient temperature. Heat rejected at a higher temperature is used as a heat source for heating buildings etc.

The concept of the heat pump has been known for many years. It operates by extracting heat that is available from a source at a particular (lower) temperature and rejecting the heat together with the energy required to drive the heat pump at a higher temperature. The rejected heat is typically used to heat a living space, such as a building, and may heat the air in the living space directly or via the use of a recirculating heat transfer fluid, such as water. A refrigerator is a type of heat pump in which the main objective is to remove heat from the object being cooled. In some circumstances, it is possible to make use of the heat rejected as well as benefiting from the refrigeration effect. Such heat pumps are particularly economic. However, in most circumstances it is difficult to match heating and cooling requirements; and in practice most systems are used either for heating or cooling.

For many years, heat pumps were not generally economic because fuel prices were low and there was little concern about emission of carbon dioxide from the burning of fossil fuel. The situation has now changed because fuel prices are rising and there is serious concern about the effects of increased concentrations of carbon dioxide in the atmosphere.

Heat pumps operating in a vapour compression cycle require a source of energy to undertake the compression stage. In an absorption type heat pump, dissolved refrigerant and solvent is pumped to high pressure as liquid and heat is used to boil refrigerant out of the solvent at a pressure under which the pure refrigerant can be condensed. The pure refrigerant is then reduced in pressure and transferred to an evaporator where it is capable of extracting heat. In an alternative arrangement, an electrically or mechanically driven compressor is used, thereby avoiding the burning of fossil fuel in the building. In vapour compression heat pumps, the amount of heat energy rejected into the building is normally several times greater than the amount of energy used to drive the heat pump. The use of heat pumps for the heating of buildings is now becoming economic and may, in some circumstances, become mandatory. A large proportion of the heat output from a heat pump is provided by the environment and is therefore renewable.

Heat pumps are known which extract heat from the ground, or from ground water. However, the present invention is concerned with heat pumps which use ambient air as the heat source. In an air-source heat pump, heat is extracted from ambient air, which heats evaporating refrigerant in the evaporator. Work is then done on the refrigerant to compress it and heat it to a higher temperature. Heat is output from the compressed refrigerant at a higher temperature and is used as a heat source for heating the building etc. Generally speaking, heat pumps are at their most efficient when the difference between the temperature at which heat is absorbed and the temperature at which heat is rejected is small. However, this is the situation in which there is least requirement for heating of the building. The greatest quantity of heat is normally required when the ambient air is at its coldest and the heat pump is therefore at its least efficient.

Another disadvantage is that if the temperature of the evaporator is allowed to become too low due to the use of particularly cold ambient air, there is a danger that the evaporator will become choked with condensed water vapour in the form of frost. Thus, it is a disadvantage of air source heat pumps that some source of supplementary heating is normally required in cold weather. For example, it is known to provide a source of auxiliary heating, but this is usually done electrically. When the ambient temperature is particularly low, an auxiliary heater can be activated to provide additional heating, usually directly into the building to augment the heat from the heat pump. As the ambient temperature becomes colder, the efficiency of the heat pump is reduced and the need for additional auxiliary heating increases.

However, it is known from U.S. Pat. No. 4,191,023 to provide an auxiliary combustion heater, which operates as part of the heat pump itself. In this case, the auxiliary heater boils refrigerant in a secondary circuit before it reaches the condenser, and increases the amount of heat rejected from the heat pump. However, such system does not address the problem of frosting in the evaporator.

The present invention seeks to mitigate these disadvantages.

Broadly speaking, the present invention relates to the use of a heater which directly heats the incoming air to an air-source heat pump and thereby prevents frosting when the ambient air temperature is low. This effectively increases the temperature of the ambient air and thereby increases the efficiency of the heat pump, as well as reducing frosting.

Specifically, the present invention provides an air-source heat pump wherein heat is extracted from air at ambient temperature by an evaporator and heat is rejected at a higher temperature in a condenser; the heat pump further comprising an auxiliary heater arranged to preheat the ambient air when required to prevent deposition of frost on the evaporator.

Generally speaking, the auxiliary heater will only be operated when there is a risk of frost forming on the evaporator surfaces, for example under conditions of low temperature. Generally speaking, the auxiliary heater will be arranged to provide auxiliary heat to the ambient air when the ambient temperature falls below 10° C., particularly below 5° C., especially below 2° C. and most especially below 1° C. When the relative humidity of the ambient air is high, the heating effect of the ambient air is greater and there is therefore less need for auxiliary heating.

The auxiliary heater will usually be arranged to operate by monitoring the evaporating pressure (or temperature) of refrigerant and then applying sufficient heat by means of the auxiliary heater to prevent the refrigerant evaporating at a pressure that would allow frost to form. Alternatively, the auxiliary heater could be controlled dependent on the temperature and humidity of the ambient air; but this is more difficult to do in practice.

The auxiliary heater may be an electrical heater or may use waste heat from other sources. However, it is particularly preferred to employ a combustion heater, wherein flue gas from the combustion heater mixes with the ambient air. The ambient air itself may be used as the air source for the combustion heater or the air may come from elsewhere. Although the combustion heater itself contributes to the carbon dioxide emission of the system, it is nevertheless a relatively cheap and efficient form of auxiliary heating and one that will not be required during most of the heating season in a temperate climate. The conditions will generally be arranged such that water as a product of combustion within the flue gases condenses as liquid on the surfaces of the evaporator and will thereby allow the latent heat of evaporation to be recovered. The auxiliary combustion heater generally uses fossil fuels, such as hydrocarbons as the fuel source. It has been found that gas (such as North Sea gas), which has a low sulphur content does not cause corrosion of the evaporator surfaces. The hydrocarbons are generally liquid or gaseous hydrocarbons, more particularly with less than 10 carbon atoms. C₁-C₆ hydrocarbons are preferred, especially natural gas (which is largely methane), propane or butane.

In a preferred embodiment, the auxiliary heater is a combustion heater (e.g. a gas-fired boiler) which heats a fluid used to provide heat directly to the building (or anything else to be heated). Typically, the fluid is air or recirculating water. The heater is operated when additional heating is required, and provides heat directly to the building via the heated fluid. Also, flue gases from the boiler are mixed with ambient air before the air is passed over the evaporator, and thus provide additional heat input to the heat pump.

The gas fired boiler may be of the condensing type, to improve efficiency and reduce the heat content of the flue gases relative to the heat content of the heated fluid. It may alternatively be of the non-condensing type.

The heated fluid is usually air or recirculating water. Other gases or liquids could be used for other applications. Recirculating water is generally used to heat radiators. The heated air or water may provide under-floor heating in a building.

The heat pump may employ any suitable refrigerant. A particular benefit of the present invention is that it allows the temperature at which heat is extracted (i.e. the evaporating temperature) to be controlled so as to improve the performance of the heat pump. The heat pump typically operates in an evaporation-condensation cycle using a source of energy to compress the volatile refrigerant. The source of energy may be a compressor (such as an electrically driven compressor) or may be a liquid pump plus a source of heat in an absorption type system.

Typically, the heat pump will be arranged to extract heat from ambient air (preheated if necessary) in the range 5-15° C., particularly 7-12° C. Heat is typically rejected at a temperature in the range 30-35° C. for direct air heating or for under-floor heating and, typically 50-55° C. for the heating of circulating water.

An embodiment of the present invention will now be described by way of example only, with reference to the attached figure, wherein

FIG. 1 is a schematic drawing of an air-source heat pump according to a first embodiment of the present invention, which utilises a combustion heater as the auxiliary heater; and

FIGS. 2 a and 2 b are schematic drawings of a second embodiment, where the auxiliary heater is a gas-boiler which also heats a flow of recirculating heating water.

Briefly, FIG. 1 shows the circuit of an air-source heat pump having a compressor 1, a condenser 2, an expansion device 3, an evaporator 4, a casing 5, a fan 6, a gas burner 7, and interconnecting piping 14. The heat pump contains a volatile refrigerant which is re-circulated.

In operation, liquified refrigerant from the condenser 2 at a temperature 55° C. is expanded through an expansion device 3 into an evaporator 4 wherein the refrigerant evaporates within the pipes of the evaporator and extracts heat from ambient air. The refrigerant at a temperature of 5° C. is drawn from the evaporator by the compressor 1, is compressed again to condensing pressure and is condensed in the condenser 2. Condensing refrigerant at a temperature of 55° C. rejects heat to the building to be heated and is thereby liquefied at a temperature of 55° C. The heat extracted from the air stream plus the equivalent of the work put into the compressor is rejected from the condenser. The refrigerant then flows through the interconnecting piping 14 back to the expansion device 3. At the expansion device the refrigerant is reduced to evaporating pressure and returned to the evaporator, where the cycle recommences.

Ambient air 10 is the source of heat for the heat pump. Ambient air is drawn into the casing 5 by means of a fan 6 and exits in the direction of the arrow. Under normal circumstances where the temperature of the ambient air is not too low, heat is extracted from the ambient air by heat exchange with the surfaces of the evaporator 4. However, when the temperature of the ambient air reduces there is the danger of frost collecting on the evaporator surfaces. The temperature and/or pressure of the evaporator may be monitored by a sensor means (not shown) and when a predetermined value is reached, the combustion heater 7 is operated. The combustion heater uses natural gas (which generally speaking does not produce corrosive flue gases which might damage the evaporator surfaces). The auxiliary combustion heater 7 is operated to bring the temperature of the ambient air back to a preset temperature, whereby the danger of frosting is avoided and the efficiency of the heat pump is improved. Water that condenses on the evaporator surfaces may be arranged to drain away via a drain (not shown).

Thus, under normal operating conditions, heat is extracted from ambient air without producing frost on the evaporator surfaces. However, when the ambient air temperature falls to such an extent that frost would otherwise form on the evaporator, the auxiliary gas burner 7 is ignited, thus providing an additional source of heat and preventing the formation of frost. The burner may be controlled to provide a preset temperature in the ambient air flowing over the evaporator surfaces, or may be controlled to prevent the refrigerant evaporating at a temperature below the freezing point of water.

EXAMPLE

Using a selected compressor, evaporator and condenser utilising ammonia as the refrigerant, it can be shown from manufacturer's data that with an air at a temperature of 15° C. (75% relative humidity), and an air flow of 17 cubic m/s, the heat extraction will be 140 kW when the refrigerant evaporates at 10° C. (6.15 bar absolute) and condenses at 55° C. Under these conditions the compressor will absorb 34.7 kW. The coefficient of performance (heating) is therefore (34.7+140)/37.4=5.02.

The limit of operation of the system before frost is formed is when the refrigerant evaporates at 0° C. (4.19 bar absolute). When refrigerant evaporates at 0° C. and condenses at 55° C. the refrigerating system will extract 101.7 kW of heat and absorb 33.7 kW of power. The coefficient of performance (heating) is therefore (33.7+101.7)/33.7=4.02.

Thus even under limiting conditions the heat pump will still produce more than four times the input energy in the form of heat.

It can be calculated that the ambient temperature is about 3° C. at the limiting condition when the refrigerant evaporates at 0° C. If ambient temperature falls below 3° C., the auxiliary heater will be engaged progressively till all the heat is being provided by the auxiliary heater.

In a temperate climate such as that of Great Britain or Japan, the temperature rarely falls below 3° C. for any considerable period of time. The invention described therefore provides an effective method of providing heat without having to use fossil fuel except in exceptional circumstances. When fossil fuel does have to be used, it is used at maximum efficiency because the products of combustion are cooled down to about 3° C.

FIGS. 2 a and 2 b show a second embodiment (analogous parts are marked with the same reference numerals as in FIG. 1) where the gas burner 7 not only provides flue gas into the evaporator 4 but also heats a recirculating flow 15 of heated water, which is used for heating the building (together with heat from the condenser 2).

FIG. 2 a shows a gas-fired boiler 7 housed in a separate chamber defined by a partition 8. Air flow 9 supplies the boiler with combustion air and flue gases 12 exit via boiler flue 13. Ambient air 10 enters the heat pump unit through louvres 16 in the wall of the casing 5, and mixes with flue gases 12 from the gas boiler. The mixed gas stream then passes over evaporator 4, as described above, and exits as stream 11.

A heated water circuit 15 is provided to heat radiators etc., within the building. It picks up heat rejected in condensor 2 during normal operation of the heat pump. However, when additional heating is required the gas-fired boiler 7 is operated. This provides additional heat to the recirculating water 15 and also provides flue gases, which provide additional heat to the ambient air 10 flowing over the evaporator 4. This provides a balance of direct heating via water stream 15 and indirect heating via the heat pump, which can be optimised to suit different conditions. 

1. An air-source heat pump wherein heat is extracted from air at ambient temperature by an evaporator and heat is rejected at a higher temperature by a condenser; the heat pump further comprising an auxiliary heater arranged to preheat the ambient air when required to prevent deposition of frost on the evaporator; the auxiliary heater being a combustion heater, and flue gases from the combustion heater mixing with the ambient air before it passes over the evaporator; and a heated fluid circuit passing through condenser and through the combustion heater.
 2. A heat pump according to claim 1, wherein the auxiliary heater is arranged to operate when the temperature of the ambient air falls below 5° C.
 3. A heat pump according to claim 2, wherein the auxiliary heater is arranged to operate when the temperature of the ambient air falls below 2° C.
 4. A heat pump according to claim 1, which is arranged to monitor the evaporating pressure or temperature and to apply sufficient heat by means of the auxiliary heater to prevent the evaporator operating at a pressure which would allow frost to form on the evaporator.
 5. A heat pump according to claim 1, arranged to operate such that water of combustion from the flue gases condenses as liquid on the surfaces of the evaporator, thereby recovering latent heat of evaporation.
 6. A heat pump according to claim 1, wherein the combustion heater is a gas fired boiler.
 7. A heat pump according to claim 6 wherein the gas fired boiler is of the condensing type.
 8. A heat pump according to claim 1 wherein heat is rejected at a temperature in the range 30-35° C. for direct air heating or under-floor heating.
 9. A heat pump according to claim 1 wherein heat is rejected at a temperature in the range 50-55° C. for heating water as the heated fluid.
 10. (canceled) 